Avian Flu is ...

Avian flu (or avian influenza, and commonly known as bird flu) is an influenza type A virus that appears in many different sub-types classified according to the nature of the two components that make up the virus - haemagglutinin (H) and neuraminidase (N). Haemagglutinin is a protein found on the surface of influenza viruses which is responsible for binding the virus to the cell that is being infected; neuraminidase is also found on the surface of influenza viruses. There are 16 haemagglutinin and 9 neuraminidase subtypes of influenza A virus, giving rise to hundreds of variations on the 'HxNy' combination. All combinations may be found in wild aquatic birds, while H1, H2 and H3 have been circulating on and off in the human population for at least one century.

Avian flu in wild and domestic birds can exist in two different forms - one that has a low capacity for causing disease (low pathogenic avian influenza or LPAI) and one that causes disease very easily (highly pathogenic avian influenza or HPAI). The form of avian flu that is currently the subject of concern is known as H5N1, which falls into the category of HPAI avian flu because it has killed tens of millions of domestic birds and probably tens of thousands of wild birds.

Migratory water fowl - most notably wild ducks - constitute the natural reservoir of the virus. Poultry flocks (chickens, ducks, turkeys, geese) are susceptible because the virus can spread rapidly through contact between a sick bird and a healthy bird. Wild birds may carry H5N1 from one area to another through the process of migration. However, the conditions in the production environment, on farms and in rice paddy fields play a major role in subsequent secondary spread of the disease, as do the carrying of poultry from one point to another and live bird markets.

Influenza A (H5N1) virus is very contagious among birds and carries a high mortality rate. In addition, if an outbreak occurs, many healthy birds risk being culled to prevent spread of the disease. As of mid-2006, it is widely estimated that at least 200 million domestic birds (out of a total world population of 10 billion) have either died or been culled as a result of H5N1.

The spread of the disease has raised great concerns for animal and public health: in the last 10 years there has been a progressive increase in the number of outbreaks of avian flu in poultry compared with the previous 40 years. While avian flu is primarily a bird disease, affecting mostly poultry and some types of wild bird, it can also affect other animals. The virus is known to have occurred in cats and related animals such as leopards and tigers, in ferrets and stone martens (one case was reported in February 2006 on the island of Ruegen in Germany) and in dogs and pigs. It is thought that other animals contract the disease through eating raw infected birds.

The disease can also affect humans but only after eating poultry meat that has not been cooked properly or after very close contact between a person and an affected animal. Even so, it is rare for the current strain of avian flu - highly pathogenic H5N1 genotype Z - to transfer from animals to humans.

However, if it infects humans, the consequences may be dramatic. The human health implications of avian flu were revealed in 1997 during outbreaks in Hong Kong. A total of 18 people fell sick, six of whom died. Since then, there have been other episodes with human deaths - in Hong Kong and in the Netherlands in 2003, and the current series of outbreaks in Asia (mostly in Southeast Asia), Central Asia, Africa and Europe, which has resulted in over 240 cases of human disease and 141 deaths since the end of 20031.

The fact that people become sick from infected poultry rather than from wild birds indicates the need to focus on disease control at source, in domestic birds only through protecting poultry can we begin to talk of protecting people.

1 Official statistics from the World Health Organisation (WHO) as of 23 August 2006